No one knows exactly but it will be not very fast as with all the other staking services if you by fast mean a considerable amount.The ETH network only with the latest upgrade if I understood it correctly lets you withdraw any staked ETH or any rewards you got from this.I think this will be more clear in the near future but many sites that I have checked are pointing toward an average of 4.52% interest but none of them shows us the time,most probably because there is still not clear even in the ETH developers,so we have to wait to know it for sure at how fast,I am assuming that 4.52% to be yearly but that is just my assumption.